Project Summary TR&D 4 proposes to construct a cryogenic recirculation system and an associated MAS probe that will permit high spinning frequency MAS experiments using He gas as a fluid in the 200-250 kHz regime. In addition, the system will function at low temperatures enabling DNP experiments with improved sensitivity. The cryogenic system will involve 4 steps as follows: (1) We plan to design and construct a recirculation system to perform MAS experiments at 80 K that will initially use N2 as a working fluid since it is less expensive and will allow us to assemble and debug the system ? insure that does not leak, etc. (2) In the second step we will introduce He into the system as the operating fluid. At ambient temperatures this will provide a means to achieve high spinning frequencies, with the eventual goal being the 200-250 kHz regime. Therefore, this system will be important in performing the experiments in TR&D 1, 2 and 4 and for almost all of the solid state NMR experiments in the DBP?s and CE?s. (3) Once this is functioning we plan to use He at ~20-50 K which will provide another factor of 2-4 in Boltzmann polarization for both conventional NMR and DNP experiments. It could also enable 1H detected DNP experiments. (4) Will construct a MAS probe for low temperature operation based on our transmission line design that has been successfully implemented twice at 500 MHz. Concurrently we plan to develop a new class of MAS rotors fabricated from single crystal diamond. Diamond is the preferred material because of its high strength, excellent thermal conductivity, and it is transparent to microwaves so that it will improve DNP enhancements. Because the rotors are small this is economically feasible. The rotors will probably be laser machined from single crystals with a femtosecond laser and if we are successful in spinning at above 100 kHz at 100 K they will open an entire new field of 1H detected DNP.